1. Field of the Invention
The present invention relates to a negative angle-forming die for forming metallic thin plates. In this context, a negative angle-forming die is used for forming in which a portion of a work is intruded into a lower die further than a descending locus in a straight direction of an upper die.
2. Description of the Background Art
Negative angle forming wherein a work of metallic thin plate is intruded into a lower die than a descending locus of an upper die in a straight manner is usually performed by using a slide cam.
A conventional method for performing intrusion forming of a work of metallic thin plate (a workpiece) included the steps of placing the work onto a lower die, descending an upper die downward in a vertical direction, driving a passive cam of the lower die by an active cam of the upper die for processing the work from a lateral direction, and after the upper die was raised upon completion of processing, the active cam was retracted by a spring.
In this case, a forming portion of the passive cam that forms the work by being slid from outside of the work in a lateral direction is uniformly formed to assume a shape that is identical to that of a forming portion of the work. However, since the work needs to be taken out from the lower die upon completion of processing, the forming portion of the lower die onto which the work is mounted is either arranged in that an intruding portion of the lower die is provided to be separable for retraction or in that a rear portion of the intruding portion is cut off and the work is moved forward for enabling the work to be taken out. While it does not become problematic in case of a minor degree of intrusion, in case such a degree is large or the work is a long and slender frame-like part having a grooved section such as a front pillar outer of metallic thin plate used in a vehicle, it may happen in case the intruding portion of the lower die is divided or cut off that, owing to the narrow groove width of the work, no clear shape can be formed by the forming portion of the passive cam and moreover, that insufficient strength of the lower die makes it impossible to perform intrusion forming at all.
Further, there are also cases in which torsions or warps are formed in products so that correction of products needs to be performed, while correction of products is practically impossible in case of parts constituting an outer panel portion of a vehicle such as a side panel, fender, roof, bonnet, trunk lid, door panel or front pillar outer all of which assume three-dimensional curved surfaces and curved lines. In case of assembling metallic thin plates for a vehicle, products which assume torsions or warps can be hardly combined to other parts, and it could not be provided for a structure of metallic thin plates for a vehicle of high quality and it could not be maintained for a specified degree of product accuracy required for the product of metallic thin plate.
For solving the above problems, it has been devised for an arrangement as follows for converting descending, straight movements of the upper die into rotational movements for rotating a columnar body, forming a forming portion that is intruded into the lower die further than a straightly descending locus of the upper die, and after completion of forming, retracting the columnar body in a rotating manner to an extent that a formed work can be taken out form the lower die.
That is, as shown in FIGS. 8 to 11, the negative angle-forming die is comprised of a lower die 102 for mounting a work W of metallic thin plate onto a supporting portion 101 thereof and an upper die 103 that is descended in a straight direction towards the lower die 102 for forming the work W by hitting against the work W, wherein there are further provided a columnar body 106 arranged in a freely rotating manner on the lower die 102 and having a groove 104 that is open to an outer peripheral surface and that is carved into an axial direction as well as an intruding forming portion 105 formed at an edge portion of the groove 104 closer to the supporting portion 101 that is intruded further than a locus of the upper die 103, a slide cam 108 formed with an intruding forming portion 107 being arranged on the upper die 103 in a freely sliding manner as to oppose the columnar body 106, and an automatic returning tool 109 arranged at the lower die 102 for retracting the columnar body 106 in a rotating manner to an extent with which the work W can be taken out from the lower die 102 upon completion of forming, wherein the work W that is mounted on the supporting portion 101 of the lower die 102 is formed through the intruding forming portion 105 of the columnar body 106 and the intruding forming portion 107 of the slide cam 108 while the columnar body 106 is rotated and the slide cam 108 performs sliding movements, and after completion of forming, the columnar body 106 is retracted in a rotating manner by the automatic returning tool 109 such that the formed work W can be taken out from the lower die 102.
Operations of such a negative angle-forming die will now be explained.
As shown in FIG. 8, the upper die 103 is first positioned at a top dead center at which time the work W is mounted on to the supporting portion 101 of the lower die 102. At this time, the columnar body 106 is retracted in a rotating manner by the automatic returning tool 109.
Next, the upper die 103 starts to descend, and as shown in FIG. 9, the slide cam 108 is first made to abut against a rotating plate 111 of the slide cam 108 without interfering the intruding forming portion 105 of the columnar body 106 so as to rotate the columnar body 106 in a rightward direction in FIG. 9 to put the columnar body 106 into a forming posture, and thereafter, a pad 110 pressurizes the work W.
When the upper die 103 keeps on descending, the slide cam 108 that is energized in a direction outside of the die moves leftward in a lateral direction of the drawing by the action of the cam against energizing force of a coil spring 112 to assume a condition as shown in FIG. 10, whereby intrusion forming of the work W is performed through the intruding forming portion 105 of the rotated columnar body 106 and the intruding forming portion 107 of the slide cam 108.
After intrusion forming is completed, the upper die 103 starts rising. The slide cam 108, being energized in a direction outside of the die by the coil spring 112, is moved rightward in FIG. 11 so that it can be raised without interfering the work W.
On the other hand, accompanying the rising movements of the slide cam 108 which had been constraining the columnar body 106, the columnar body 106 rotates leftward in FIG. 11 through the automatic returning tool 109. In this manner, when taking the work W out from the lower die 102 after intrusion forming, the work W can be taken out without interfering the intruding forming portion 105 of the columnar body 106.
In the above-described negative angle-forming process, a lower surface of the slide cam 108 that is energized by the coil spring 112 abuts against the rotating plate 111 that is energized by a coil spring 113 of the automatic returning tool 109 to rotate the columnar body 106 in a rightward direction such that the columnar body 106 assumes a forming posture whereupon the pad 110 pressurizes the work W. However, drawbacks are presented in that negative angle-forming processes of favorable quality cannot be performed in case the work W in the forming posture is rotated to some extent in the leftward direction in the drawing since the energizing force applied to the work W that is mounted on the pad 110 is too large, or in case the energizing force of the coil spring 112 of the slide cam 108 and that of the coil spring 113 of the automatic returning tool 109 are not balanced, whereby the columnar body 106 is slightly rotated such that it cannot maintain a specified forming posture to consequently result in a case in which a stepped portion is formed in a curved surface of the work W or in which no accurate straight line can be formed. Consequently, it becomes difficult to provide products of accuracy in units of 1/100 mm, and negative angle forming of favorable quality cannot be performed.
As it can be seen from FIGS. 8 to 11, the columnar body 106 is supported by the lower die 102 by directly contacting thereat except for the portion of the groove 104 whereby it is required to accurately perform processing of the columnar body 106 and a portion of the lower die 102 for supporting the columnar body 106 (a hole having a substantially circular section) so as to make processing difficult.
Moreover, the negative angle-forming die assumes a large dimension owing to its arrangement in which almost the entire outer periphery of the columnar body 106 is supported by the lower die 102 and the rotating plate 111 is formed as to extrude from the columnar body 106, whereby the mold becomes quite expensive.